Apparatus for making a web-supported membrane

ABSTRACT

A process and apparatus for continuously forming a reinforced membrane of fluorinated polymers containing sulfonyl and/or carboxyl groups in melt-fabricable form and, after hydrolysis, the corresponding membrane in ion exchange form. The membrane thus formed has exceptional uniformity, and this membrane so made, when used as the membrane which separates the compartments of a chloroalkali cell, offers the advantages of low operating voltage, low power consumption and high current efficiency, and a long useful life without rupture.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of my prior copending United Statesapplication Ser. No. 304,105 filed Sept. 21, 1981, which in turn is adivision of my prior United States application Ser. No. 121,461 filedFeb. 14, 1980, now U.S. Pat. No. 4,324,606, which in turn is acontinuation-in-part of my prior application U.S. Ser. No. 107,521 filedDec. 27, 1979, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to an improved cationic exchange membrane, aprocess for making that membrane, and apparatus which is used in theprocess. More particularly, the membrane is a fluorinated cationexchange membrane, one important use of which is to separate the anodeand cathode compartments of a chloralkali cell.

Fluorinated polymers containing pendant side chains containing sulfonylgroups are now well-known, and their use as ion exchange membranes isalso known. It is desirable to have an ion exchange membrane which issupported, i.e., which contains a material which imparts physicalstrength to the fluorinated polymer, so that the physical strength ofthe complete membrane construction is greater than that of a film of thefluorinated polymer. Heretofore, methods for supporting such membraneshave not been adequate, for if films of desirable thickness wereemployed complete encapsulation of the support material was noteffected, and if complete encapsulation of the support material were tobe assured excessively thick films of fluorinated polymer were required.Such excessive film thickness not only increases the cost of themembrane, but it also reduces the usefulness of the membrane for ionexchange purposes because the increased thickness leads to higheroperating voltage and higher power consumption. If the support materialis not completely encapsulated, the membrane will leak or will in useultimately rupture at the non-encapsulated points and will then leak,and thus its usefulness is impaired.

A method which has been proposed to overcome the above problems anddeficiencies is that of U.S. Pat. No. 3,770,567 wherein a film offluorinated polymer which contains pendant side chains containing --SO₂L groups, where L is F or Cl, is treated on one surface with an alkalimetal hydroxide, an alkaline earth metal hydroxide or ammoniumhydroxide, to form a hydrolyzed surface layer wherein the functionalgroups are in the --(SO₃)_(j) M form, where M is alkali metal, alkalineearth metal or ammonium, and j is the valence of M, followed bycontacting the --SO₂ L surface of the film with a support material, andapplying a differential pressure to the contacted support material andthe film, the pressure on the opposite surface of the support materialfrom that which is contacting the fluorinated polymer being at least 5inches (127 mm) of mercury less than the pressure on the surface of thefluorinated polymer film opposite to that contacting the supportmaterial, for a sufficient period of time to cause the support materialwhich is in contact with said film to become completely encapsulatedwithin the film of fluorinated polymer while heating the film andsupport material at a temperature of from 240°-320° C. The resultinglaminate is then subjected to a second hydrolysis treatment with alkalimetal hydroxide, alkaline earth metal hydroxide or ammonium hydroxideafter which it is ready for use for ion exchange purposes. This methodhas the disadvantage of adding an additional processing step in theformation of the supported structure, and many additional hours ofprocessing time are required to effect the surface hydrolysis step inthe hydroxide treating bath used. Additionally this method cannot beused for fluorinated polymers which contain carboxylic functional groupsbecause the hydrolysis step would lead to carboxylic acid groups orsalts thereof, which easily decarboxylate at the temperatures employedin forming the supported construction.

It is a principal object of the invention to provide novel web supportedmembranes of exceptional uniformity. In the novel web reinforcedmembranes the sulfonyl and carboxyl groups can be either in meltfabricable form, or, after hydrolysis or other suitable chemicalreaction, can be in ion exchange form.

It is another object of this invention to provide a process for forminga supported structure of fluorinated polymers which contain pendant sidechains containing either sulfonyl groups or carboxyl groups or both,which method leads to a completely encapsulated supported structure, andwhich eliminates the necessity for a surface hydrolysis step.

It is a further object to provide apparatus adapted for carrying out theprocess of the invention specified immediately above.

Other objects will be apparent from the continuing description.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a reinforcedmembrane which consists essentially of at least two layers of meltfabricable fluorinated polymer which contains side chains containingsulfonyl and/or carboxyl groups in melt fabricable form, and a wovenreinforcing fabric comprising warp and fill strands, there being atleast one layer of a said fluorinated polymer on each side of saidreinforcing fabric, the warp and fill strands of said reinforcing fabricdefining windows between said strands, each layer of fluorinated polymerin at least 70% of the area in each of at least 90% of said windowsbeing of uniform thickness within plus or minus 10%.

There is also provided according to the invention reinforced membranesin ion exchange form made by hydrolyzing or otherwise chemicallymodifying such melt fabricable membranes.

There is additionally provided according to the invention a process forcontinuously forming a reinforced membrane comprising

(1) continuously bringing at least two films of melt-fabricablefluorinated polymer which contain side chains containing sulfonyl and/orcarboxyl functional groups in melt fabricable form and a web ofreinforcing material into face-to-face contact such that proximatesurfaces of two of said films contact opposite planar surfaces of saidweb, and moving the resulting combination of said films and said webvertically, unsupported except at two opposite edges thereof,

(2) removing air from between said films at the two opposite edgeportions thereof,

(3) applying heat to the two outermost opposite planar film surfaces,first in the center portions thereof and progressively moving toward andincluding the edge portions thereof, and

(4) cooling the resulting reinforced membrane.

There is further provided according to the invention apparatus formaking a reinforced membrane, comprising,

(1) a frame, and mounted on said frame,

(2) means for guiding at least two continuous webs of film and acontinuous web of reinforcing material into face-to-face contact suchthat proximate surfaces of two of said webs of film contact oppositesides of said web of reinforcing material,

(3) two sets of flexible endless belts which cooperate to engageopposite sides of the resulting assembly of said webs at the edgeportions thereof and to transport said assembly, each set consisting oftwo belts, one belt of each set having a series of perforations alongits entire length, each set of belts extending beyond an edge of saidassembly, and guide means for sad belts,

(4) vacuum means including two vacuum manifolds, one manifold adjacenteach said perforated belt, for removing air from between said films ofsaid assembly at the edge portions thereof through said perforations,

(5) two banks of heaters, one bank adjacent each exposed film surface,each bank consisting of a plurality of heaters disposed for heatingfirst the center portion of said assembly and then progressively towardand including the edges thereof as said assembly is transportedtherebetween, to fuse said assembly into a reinforced membrane,

(6) means for guiding said assembly between said banks of heaters,

(7) a wind-up for collecting said reinforced membrane, and

(8) means for driving said wind-up.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts schematically a side view of the apparatus of theinvention.

FIG. 2 depicts schematically a front view of a portion of the apparatusof the invention.

FIG. 3 is a pictorial view of a detail at one edge portion of theapparatus of the invention.

FIG. 4 is a photograph, at a magnification of 60, of a cross-section ofa membrane of the invention, cut in a direction parallel to the machinedirection of the lamination process.

FIG. 5 is a photograph, at a magnification of 60, of a cross-section ofthe same membrane as in FIG. 4, but cut in a direction perpendicular tothe machine direction of the lamination process.

DETAILED DESCRIPTION OF THE INVENTION

Referring first to FIG. 1, it should be understood that the variouselements of the apparatus of the invention are joined either directly orindirectly to a frame 1 by means of various supports, of which supports2 and 3 are typical, the remaining supports not being shown forsimplicity.

Melt fabricable films 11 and 12 of fluorinated polymer containingsulfonyl and/or carboxyl groups, to be described more fully below, and aweb of reinforcing material 13, also to be described more fully below,are unwound from supply rolls 14, 15 and 16, respectively, in thedirection of the arrows shown. Films 11 and 12 and web 13 are broughttogether over guide roll 17, and further guided through the apparatusaround guide rolls 18 and 19.

Flexible belts 20 and 21 cooperate to contact opposite sides of one edgeportion of the assembly of films 11 and 12 and web 13. Belts 20 and 21are suitably made of a thin impervious material such as stainless steelabout 5 to 20 mils (0.13 to 0.5 mm) thick. Belts 20 and 21 move in thedirection of the arrows shown.

Belt 20 is guided in a path around rolls 18 and 22. Roll 22 is mountedso as to be movable vertically by means of air-actuated cylinders, notshown. In this way roll 22 presses against belt 20 to create a positivetension which prevents the belt from slipping on the rolls it contacts.

Belt 20 is restrained from wandering laterally along the indicated rollsby use of two guide blocks 23 and 24 within which it passes just beforeit contacts rolls 18 and 22, respectively. Each guide block isfabricated from two members (not separately shown) held together byscrews. The first member is in the shape of a rectangular solid havingmachined in one face a shallow depression, about 0.25 mm (10 mils) deepfor a belt 0.2 mm (8 mils) thick, and about 1/16 inch (about 1.5 mm)wider than the width of belt 20, and the second member in the form of arectangular solid is secured by screws to the first member over themachined depression to form a slot through which belt 20 passes. Thesecond member of the guide block is placed in position only afterendless belt 20 has been positioned in the machined depression. Guideblocks 23 and 24 are suitably fabricated from aromatic polyimide resinfilled with graphite as is disclosed in U.S. Pat. No. 3,179,631.

Belt 20 contains a series of perforations along its entire length, notshown in FIG. 1, but which will be more fully explained below.

Belt 21 is guided in a path around rolls 17, 18, 19, 25, 26 and 27. Belt21 is restrained from wandering laterally along those rolls by use offlanges between which belt 21 runs on rolls 26 and 27.

The shaft of roll 26 is mounted with a screw adjustment for moving roll26 toward and away from belt 21 to permit increasing or decreasing thetension on belt 21. In this way the tension on belt 21 can be adjustedso that it will not slip.

A second set of belts, not shown in FIG. 1 but identical to belts 20 and21, similarly grips the second edge portion of the assembly of films 11and 12 and web 13.

Roll 28 is a resilient rubber roll which is urged by air actuatedcylinders toward roll 18 to press belts 20 and 21, films 11 and 12, andweb 13 into intimate contact and against roll 18.

A vacuum manifold 29, whose operation will be more fully explainedbelow, is positioned adjacent perforated belt 20. Pipe 30 serves to joinvacuum manifold 29 to a vacuum source not shown. A second vacuummanifold, not shown in FIG. 1 but identical to vacuum manifold 29, ispositioned adjacent to the second perforated belt referred to above butalso not shown in FIG. 1.

A sealing shoe 35 to be described in detail below presses against belt21 and urges belts 20 and 21 films 11 and 12 and web 13 toward vacuummanifold 29. A similar sealing shoe is used in cooperation with thesecond set of belts not shown in this view.

Two banks of heaters (not shown in FIG. 1) contained within housings 31and 32 are positioned on opposite sides of the assembly of films 11 and12 and web 13. The heaters serve to soften films 11 and 12 sufficientlyto permit the assembly of films 11 and 12 and web 13 to be fused into amembrane. The membrane 33 thus formed is collected on wind-up 34.

Wind-up 34 and rolls 17 and 18 are driven. They provide the drivingforce for moving the membrane and assembly of films 11 and 12 and web 13through the apparatus. The remaining rolls in the apparatus all can beidler rolls.

Rolls 17 and 18 are suitably covered with a textured surface such as afine emery cloth. This provides a high enough coefficient of friction ofthe roll against films 11 and 12 to prevent the films and also belts 20and 21 from slipping against these driven rolls.

Certain elements in FIG. 1 are drawn as not in contact with one anotherfor clarity in showing them, whereas it should be understood that theyare in contact during operation. Thus, in FIG. 1, vacuum manifold isshown as not in contact with belt 20, the latter is shown as not incontact with the assembly of webs 11, 12 and 13, the latter is shown asnot in contact with belt 21, and the latter is shown as not in contactwith sealing shoe 35, whereas during operation these pairs of elementsare in fact in contact.

In FIG. 2, a partial front view of the apparatus, are seen rolls 18, 19and 22, with belt 20 shown passing over rolls 18 and 22 and beingrestrained from lateral movement by guide block 23. A second similarbelt 42, not seen in FIG. 1, passes over rolls 18 and 41, and is alsoseen in this view. Belt 42 also is restrained from lateral movement by aguide block 42. The second guide block 24 for belt 20, and a similarsecond guide block for belt 42 are not shown in this view.

Roll 41 is mounted to be movable vertically in the same manner as isroll 22. Both rolls 22 and 41 are made of resilient rubber.

Belt 20 has a series of perforations 44 along the entire length thereof,only some of which are shown in FIG. 2. Similarly, belt 42 has a seriesof perforations 45 along the entire length thereof, only some of whichare shown in FIG. 2. Housing 31 contains a bank of heating elements 46,which typically may be radiant or infrared heating elements. Housing 32,not shown in FIG. 2, contains a similar bank of heating elements. In theapparatus and process here depicted, the assembly of films andreinforcing material to be fabricated into a membrane is transportedupwardly from roll 18 and around roll 19. Heater elements 46 are arrayedin a chevron-shaped array for the purpose of first heating the centerportion of the assembly and then progressively laterally in bothdirections to and including the edge portions of the assembly, and it isfor this reason that the center of the chevron-shaped array pointsdownward. In this manner, entrapment of air bubbles between the films asthey are laminated is less likely to occur.

In the pictorial view of FIG. 3 is found a view of belts 20 and 21 andan edge of films 11 and 12 and reinforcing material 13, with eachelement in turn broken away so as to reveal the relationship of theseelements to one another. As seen in FIG. 3, an edge of each of films 11and 12 and of reinforcing material 13 extends between belts 20 and 21.Belts 20 and 21 are suitably of the same width, typically 5-15centimeters (2-6 inches) and are positioned to be in register with oneanother. As indicated above, belt 20 has a series of perforations 44along the entire length thereof.

The combination of belts 20 and 21 and assembly of films 11 and 12 andreinforcing material 13 moves upwardly, with belt 20 moving slidablyagainst and in contact with vacuum manifold 29.

Vacuum manifold 29 comprises a body 51 and a face plate 52. Body 51 isfabricated to have a wide deep recess along most of its length, exceptfor the extreme end portions thereof which are not recessed, the recessbecoming interior chamber 53 of the manifold, and is suitably made of ametal such as aluminum. Face plate 52 is fabricated to have a slot 54along most of its length, except for the extreme end portions thereofwhich are not slotted, the slot communicating with chamber 53, and issuitably made of a graphite-filled phenolic resin so that belt 20 willeasily slide against it even when drawn against it by vacuum and pressedagainst it by sealing shoe 35. The body 51 and face plate 52 are heldtogether by screws 55, and sealed together with a silicone rubbercement. Interior chamber 53 is connected to vacuum means, as wasexplained in relation to FIG. 1 above. Belt 20 and vacuum manifold 29are so positioned that perforations 44 and slot 54 are in register withone another. Typically, perforations 44 are about 6 mm in diameter andslot 54 is about 3 mm wide. Sealing shoe 35 is urged against belt 21 bya series of spring-loaded brackets, of which bracket 56 is typical.Bracket 56 is secured to the body 51 of vacuum manifold 29 by a screw57. Bracket 56 is linked to, but spaced apart from, sealing shoe 35 by abolt 58 and nuts not shown. A spring 59 surrounding the shaft of bolt 58urges sealing shoe 35 against belt 21.

In operation, when a vacuum is drawn in vacuum chamber 53, air is drawnthrough slot 54. This in turn draws belt 20 into contact with face plate52 of vacuum manifold 29. As belt 20 moves upward, it moves in slidingcontact with face plate 52. In turn, all of films 11 and 12, reinforcingmaterial 13, and belt 21 are drawn into contact with one another andwith belt 20. The outermost portions 60 and 61 of belts 20 and 21,respectively, form a temporary seal against one another. Similarly, film11 forms a temporary seal against the inner portion 62 of belt 20, thefilm 12 forms a temporary seal against the inner portion 63 of belt 21.Residual air trapped between films 11 and 12 is then drawn from betweenthem, between the strands of reinforcing material 13, and throughperforations 44 and slot 54 into vacuum manifold 29. Sealing shoe 35presses against the edge portions of belt 21 and thus aids inestablishing and maintaining the temporary vacuum seals. In this wayfilms 11 and 12 are brought into intimate contact with reinforcingmaterial 13, and are sealed into intimate contact with it as theassembly of films and reinforcing material passes between the heaters.

Films 11 and 12 are suitably of the same width and are passed throughthe apparatus in register with one another. However, film 12 may bewider than film 11 without impairing the operation of the apparatus andremoval of the entrapped air from between the films.

The web supported membranes of the invention are characterized by havingexceptional uniformity. Woven reinforcing fabrics comprise warp and fillstrands which meet at crossover points termed junctions, and whichdefine openings between the strands termed windows. In the membranes ofthe invention each layer of fluorinated polymer in at least 70%,preferably 75%, of the area in each of at least 90%, preferably 95%, ofthe windows is of uniform thickness within plus or minus 10%, preferablyplus or minus 5%. Additionally, it is preferred that the total thicknessof fluorinated polymer which covers both sides of the strands is atleast 80%, preferably 85%, of the average total thickness of the polymerin the portion of the windows which is uniform within plus or minus 10%.

FIGS. 4 and 5 are photographs at 60X magnification of cross-sections ofa membrane of the invention cut in directions parallel andperpendicular, respectively, to the machine directions of the laminationprocess. The membrane was made by laminating a film 0.051 mm (2 mils)thick of a copolymer of perfluoro (3,6-dioxa-4-methyl-7-octenesulfonylfluoride) and tetrafluoroethylene having an equivalent weight of 1100 toeach side of a fabric of fluorocarbon polymer filaments (0.127 mm, or 5mil, diameter) in a Leno weave having 68% open area. This membrane issimilar to that first prepared in Example 1 below, before the ethylenediamine treatment of that example.

The membranes of the invention are prepared from component polymer filmswhich have a thickness ranging from as low as 0.013 mm (0.5 mil) up toseveral mils. As the membrane will generally be prepared from two orthree such polymer films, the thickness of polymer in the resultingmembrane will generally lie in the range of about 0.025 to 0.25 mm (1 to10 mils).

Membranes can be made using either the same or different thickness ofpolymer on each side of the reinforcing material. In some cases thepolymer on one side of the reinforced material will be made up of layersof two different polymer films, which can be blocked together before thelamination process is carried out. During lamination the bank of heaterson one side of the assembly of webs to be laminated can be heated to ahigher or lower temperature than the bank of heaters on the oppositeside, or both banks of heaters can be heated to the same temperature.

When the same thickness of the same or similar polymer films are used oneach side of the reinforcing material and the heaters are operated atthe same temperature, the polymer films are drawn substantially equallyinto the window areas of the reinforcing material during fabrication,and the two opposite surfaces of the resulting membrane are thereforesimilarly contoured. When the heaters are operated at differenttemperatures, the film on the side heated to the lower temperature isless easily drawn into the window areas, so the contour of the surfaceof the resulting membrane on that side will be smoother or flatter thanthe contour on the opposite side which was heated to the highertemperature.

When different thicknesses of polymer film are used on opposite sides ofthe reinforcing material, and the heaters are operated at the sametemperature, the side having the greater thickness of polymer will beless easily drawn into the window areas of the reinforcing material, andthe surface of that side will have a smoother or flatter contour thanthe contour of the opposite side having a thinner layer of polymer. Whendifferent thicknesses of polymer are used on the two opposing sides ofthe reinforcing material and it is desired to have a membrane havingapproximately the same surface contour on each side, the bank of heaterson the side having the greater thickness of polymer can be heated to asomewhat higher temperature so as to permit the polymer on both sides ofthe reinforcing material to be equally drawn into the window areas.

Among the preferred membrane constructions are those having polymer withsulfonic functional groups on both sides of the reinforcing material,those having polymer with carboxylic functional groups on both sides ofthe reinforcing material, those having polymer with sulfonic functionalgroups on one side of the reinforcing material and carboxylic functionalgroups on the other side of the reinforcing material, and those havingpolymer with sulfonic functional groups on one side of the reinforcingmaterial and a layer of polymer with sulfonic functional groups and alsoa layer of polymer with carboxylic functional groups on the other sideof the reinforcing material such that the polymer with carboxylicfunctional groups constitutes the exposed surface layer.

A problem which is sometimes encountered with a chloralkali cell whichemploys a membrane to separate the anode and cathode compartments is gasblinding on the cathode side of the membrane. One of the advantages ofthe present invention is its ability to make membranes having a smootheror flatter surface on one side of the membrane as compared to the otherside. A membrane prepared to have one relatively smooth or flat side canbe positioned in a chloralkali cell such that the smoother side facestoward the cathode so as to alleviate the problem of entrapment of gasbubbles in recesses on the membrane surface facing the cathode.Membranes fabricated to have two different polymers on the two differentsurfaces, for example polymer having sulfonic acid or salt functionalgroups on one surface and polymer having sulfonamide functional groupsor carboxylic functional groups on the other surface are generallypositioned in a chloralkali cell with the latter surface facing towardthe cathode, inasmuch as that polymer composition is more effective insuppressing back-migration of hydroxyl ion through the membrane. In suchmembranes it is therefore preferred that the membrane surface havingcarboxylic or sulfonamide functional groups having a flatter contourthan the other surface.

The melt-fabricable polymer having sulfonyl functional groups istypically a polymer having a fluorinated hydrocarbon backbone chain towhich are attached the functional groups or pendant side chains which inturn carry the functional groups. The pendant side chains can contain,for example, ##STR1## groups wherein R_(f) is F, C1, or a C₁ to C₁₀perfluoroalkyl radical. Ordinarily, the functional group in the sidechains of the polymer will be present in terminal ##STR2## groups.

Examples of fluorinated polymers of this kind are disclosed in U.S. Pat.Nos. 3,282,875, 3,560,568 and 3,718,627. More specifically, the polymerscan be prepared from monomers which are fluorinated or fluorinesubstituted vinyl compounds. The polymers are made from at least twomonomers, with at least one of the monomers coming from each of the twogroups described below.

The first group is fluorinated vinyl compounds such as vinyl fluoride,hexafluoropropylene, vinylidene fluoride, trifluoroethylene,chlorotrifluoroethylene, perfluoro(alkyl vinyl ether),tetrafluoroethylene and mixtures thereof. In the case of copolymerswhich will be used in electrolysis of brine, the precursor vinyl monomerdesirably will not contain hydrogen.

The second group is the sulfonyl-containing monomers containing theprecursor group ##STR3## wherein R_(f) is as described above. Additionalexamples can be represented by the general formula CF₂ ═CF--T_(k) --CF₂SO₂ F wherein T is a bifunctional fluorinated radical comprising 1 to 8carbon atoms, and k is 0 or 1. Substituent atoms in T include fluorine,chlorine, or hydrogen, although generally hydrogen will be excluded inuse of the copolymer for ion exchange in a chloralkali cell. The mostpreferred polymers are free of both hydrogen and chlorine attached tocarbon, i.e., they are perfluorinated, for greatest stability in harshenvironments. The T radical of the formula above can be either branchedor unbranched, i.e., straight-chain, and can have one or more etherlinkages. It is preferred that the vinyl radical in this group ofsulfonyl fluoride containing comonomers be joined to the T group throughan ether linkage, i.e., that the comonomer be of the formula CF₂═CF--O--T--CF₂ --SO₂ F. Illustrative of such sulfonyl fluoridecontaining comonomers are CF₂ ═CFOCF₂ CF₂ SO₂ F, ##STR4##

The most preferred sulfonyl fluoride containing comonomer isperfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride), ##STR5##

The sulfonyl-containing monomers are disclosed in such references asU.S. Pat. Nos. 3,282,875, 3,041,317, 3,718,627 and 3,560,568.

A preferred class of such polymers is represented by polymers having therepeating units ##STR6## wherein m is 3 to 15,

n is 1 to 10,

p is 0, 1 or 2,

the X's taken together are four fluorines or three fluorines and onechlorine,

Y is F or CF₃, and

R_(f) is F, Cl or a C₁ to C₁₀ perfluoroalkyl radical.

A most preferred copolymer is a copolymer of tetrafluoroethylene andperfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) which comprises20 to 65 percent, preferably, 25 to 50 percent by weight of the latter.

When used in a film or membrane to separate the anode and cathodecompartments of an electrolysis cell, such as a chloralkali cell, thepolymer after conversion to ionizable form should have a total ionexchange capacity of 0.5 to 1.6 meq/g (milliequivalents/gram),preferably from 0.8 to 1.2 meq/g. Below an ion exchange capacity of 0.5meq/g, the electrical resistivity becomes too high, and above 1.6 meq/gthe mechanical properties are poor because of excessive swelling of thepolymer. The values of m and n in the above formula of the copolymershould be adjusted or chosen such that the polymer has an equivalentweight no greater than about 2000, preferably no greater than about1600, for use as an ion exchange barrier in an electrolytic cell. Theequivalent weight above which the resistance of a film or membranebecomes too high for practical use in an electrolytic cell variessomewhat with the thickness of the film or membrane. For thinner filmsor membranes, equivalent weights up to about 2000 can be tolerated. Formost purposes, however, and for films of ordinary thickness, a value nogreater than about 1600 is preferred.

Such copolymers used in the present invention can be prepared by generalpolymerization techniques developed for homo- and copolymerizations offluorinated ethylenes, particularly those employed fortetrafluoroethylene which are described in the literature. Nonaqueoustechniques for preparing the copolymers include that of U.S. Pat. No.3,041,317, that is, by the polymerization of a mixture of the majormonomer therein, such as tetrafluoroethylene, and a fluorinated ethylenecontaining a sulfonyl fluoride group in the presence of a free radicalinitiator, preferably a perfluorocarbon peroxide or azo compound, at atemperature in the range 0°-200° C. and at pressures in the range 1-200or more atmospheres. The nonaqueous polymerization may, if desired, becarried out in the presence of a fluorinated solvent. Suitablefluorinated solvents are inert, liquid, perfluorinated hydrocarbons,such as perfluoromethylcyclohexane, perfluorodimethylcyclobutane,perfluorooctane, perfluorobenzene and the like, and inert, liquidchlorofluorocarbons such as 1,1,2-trichloro-1,2,2-trifluoroethane, andthe like.

Aqueous techniques for preparing the copolymer include contacting themonomers with an aqueous medium containing a free-radical initiator toobtain a slurry of polymer particles in non-water-wet or granular form,as disclosed in U.S. Pat. No. 2,393,967, or contacting the monomers withan aqueous medium containing both a free-radical initiator and atelogenically inactive dispersing agent, to obtain an aqueous colloidaldispersion of polymer particles, and coagulating the dispersion, asdisclosed, for example, in U.S. Pat. Nos. 2,559,752 and 2,593,583.

The melt-fabricable polymer having carboxylic functional groups istypically a polymer having a fluorinated hydrocarbon backbone chain towhich are attached the functional groups or pendant side chains which inturn carry the functional groups. The pendant side chains can contain,for example ##STR7## groups wherein Z is F or CR₃, t is 1 to 12, and Wis --COOR or --CN, where R is lower alkyl. Ordinarily, the functionalgroup in the side chains of the polymer will be present in terminal##STR8## groups. Examples of fluorinated polymers of this kind aredisclosed in British Pat. No. 1,145,445 and U.S. Pat. No. 3,506,635.More specifically, the polymers can be prepared from monomers which arefluorinated or fluorine substituted vinyl compounds. The polymers areusually made from at least two monomers. At least one monomer is afluorinated vinyl compound from the first group described hereinabove inreference to polymers containing --SO₂ F groups. Additionally, at leastone monomer is a fluorinated monomer which contains a group which can behydrolyzed to a carboxylic acid group, e.g., a carboalkoxyl or nitrilegroup, in a side chain as set forth above. Again in this case, as in thecase of the polymers having --SO₂ F groups, the monomers, with theexception of the R group in the --COOR, will preferably not containhydrogen, especially if the polymer will be used in the electrolysis ofbrine, and for greatest stability in harsh environments, most preferablywill be free of both hydrogen and chlorine, i.e., will beperfluorinated; the R group need not be fluorinated as it is lost duringhydrolysis when the functional groups are converted to ion-exchangegroups.

One exemplary suitable type of carboxyl-containing monomer isrepresented by the formula ##STR9## wherein R is lower alkyl,

Y is F or CF₃, and

s is 0, 1 or 2.

Those monomers wherein s is 1 are preferred because their preparationand isolation in good yield is more easily accomplished than when s is 0or 2. The compound ##STR10## is an especially useful monomer. Suchmonomers can be prepared, for example, from compounds having the formula##STR11## wherein s and Y are as defined above, by (1) saturating theterminal vinyl group with chlorine to protect it in subsequent steps byconverting it to a CF₂ Cl--CFCl--group; (2) oxidation with nitrogendioxide to convert the --OCF₂ CF₂ SO₂ F group to an --OCF₂ COF group;(3) esterification with an alcohol such as methanol to form an --OCF₂COOCH₃ group; and (4) dechlorination with zinc dust to regenerate theterminal CF₂ ═CF--group. It is also possible to replace steps (2) and(3) of this sequence by the steps (a) reduction of the --OCF₂ CF₂ SO₂ Fgroup to a sulfinic acid, --OCF₂ CF₂ SO₂ H, or alkali metal or alkalineearth metal salt thereof by treatment with a sulfite salt or hydrazine;(b) oxidation of the sulfinic acid or salt thereof with oxygen orchromic acid, whereby --OCF₂ COOH groups or metal salts thereof areformed; and (c) esterification to --OCF₂ COOCH₃ by known methods; thissequence is more fully described in U.S. Ser. No. 789,726 in the namesof W. G. Grot, C. J. Molnar and P. R. Resnick, filed Apr. 20, 1977.Preparation of copolymers thereof is described in U.S. Ser. No. 789,727in the names of C. J. Molnar and P. R. Resnick, filed Apr. 20, 1977.

Another exemplary suitable type of carboxyl-containing monomer isrepresented by the formula ##STR12## wherein V is --COOR or --CN,

R is lower alkyl,

Y is F or CF₃,

Z is F or CF₃, and

s is 0, 1 or 2.

The most preferred monomers are those wherein V is --COOR wherein R islower alkyl, generally C₁ to C₅, because of ease in polymerization andconversion to ionic form. Those monomers wherein s is 1 are alsopreferred because their preparation and isolation in good yield is moreeasily accomplished than when n is 0 or 2. Preparation of those monomerswherein V is --COOR where R is lower alkyl, and copolymers thereof, isdescribed in U.S. Pat. No. 4,131,740. The compounds ##STR13## whosepreparation is described therein, are especially useful monomers.Preparation of monomers wherein V is --CN is described in U.S. Pat. No.3,852,326.

Yet another suitable type of carboxyl-containing monomer is that havinga terminal of --O(CF₂)_(v) COOCH₃ group where v is from 2 to 12, such asCF₂ ═CF--O(CF₂)₃ COOCH₃ and CF₂ ═CFOCF₂ CF(CF₃)O(CF₂)₃ COOCH₃.Preparation of such monomers and copolymers thereof is described inJapanese Patent Publications 38586/77 and 28486/77, and in British Pat.No. 1,145,445.

Another class of carboxyl-containing polymers is represented by polymershaving the repeating units ##STR14## wherein q is 3 to 15,

r is 1 to 10,

s is 0, 1 or 2,

t is 1 to 12,

the X's taken together are four fluorines or three fluorines and onechlorine,

Y is F or CF₃,

Z is F or CF₃, and

R is lower alkyl.

The values and preferred values of the ion exchange capacity andequivalent weight of the carboxylic containing copolymer should be inthe same ranges as set forth above for copolymers containing sulfonylgroups in ionizable form. Similarly, the same polymerization techniquesas set forth above are also suitable.

A copolymer which contains different types of functional groups can alsobe used as one of the component films in making the membrane of theinvention. For example, a terpolymer prepared from a monomer chosen fromthe first group described above, a monomer from the second groupdescribed above, and additionally a monomer of the carboxylic type fromthe third group described above can be prepared and used as one of thefilm components in making the membrane.

It is further possible to use as one of the component films of themembrane a film which is a blend of two or more polymers. For example, ablend of a polymer having sulfonyl groups in melt-fabricable form with apolymer having carboxyl groups in melt-fabricable form can be preparedand used as one of the component films of the membrane of thisinvention.

It is additionally possible to use a laminar film as one of thecomponent films in making the membrane. For example, a film having alayer of a copolymer having sulfonyl groups in melt-fabricable form anda layer of a copolymer having carboxyl groups in melt-fabricable form,can also be used as one of the component films in making the membrane ofthe invention.

In the process of the invention, temperatures of about 240° C. to 320°C. are ordinarily required to fuse the polymer films employed, so as toform a unitary membrane structure with the support material, and, whenmore than two films are used, to make adjacent sheets of film fusetogether; the temperature required may be even above or below thisrange, however, and will depend on the specific polymer or polymersused. Actual heater temperatures as measured by a thermocouple in theheater itself will be higher than the indicated film temperature, due toheat losses; heater temperatures will vary with the type of heater, itsplacement, etc., and will generally fall in the range of 350° to 500° C.In the apparatus described herein, heater temperatures of 400° to 475°C. have been found suitable for making many membranes. The choice of asuitable temperature in any specific case will be clear, inasmuch as toolow a temperature will fail to effect an adequate degree of adherence ofthe films to the reinforcement member and to each other, and too high atemperature will cause gas bubbles to form between or within the polymerfilms in the window areas and will cause the films to sag and formnonuniform layers.

The reinforcement fabric for encapsulation within the membrane issuitably a woven or nonwoven fabric. Woven fabrics include those ofordinary weave, and warp knit fabrics. A woven fabric is preferred for amembrane to be used in an electrolysis cell. Fabrics prepared fromeither monofilament or from multistranded yarns can be used. The fabricshould be able to withstand a temperature from about 240° C. to about320° C., since these temperatures are employed in the laminating steps.With this proviso, the individual reinforcing fibers can be made fromconventional materials, since their main purpose is to strengthen themembrane. Due to chemical inertness, reinforcement materials made fromperfluorinated polymers have been found to be preferred. The polymersinclude those made from tetrafluoroethylene and copolymers oftetrafluoroethylene with hexafluoropropylene and perfluoro (alkyl vinylethers) with alkyl being 1 to 10 carbon atoms such as perfluoro (propylvinyl ether). An example of a most preferred reinforcement material ispolytetrafluoroethylene. Supporting fibers made fromchlorotrifluoroethylene polymers are also useful. Other suitablereinforcing materials include quartz and glass. Such reinforcementmaterials and their use to strengthen polymers in a membrane are wellknown in the prior art.

Other reinforcing members in sheet-like form can also be used in theprocess of the invention. These include various metals and alloys suchas stainless steel and titanium in the form of screen or expanded mesh,sheets of perhalogenated polymers such as polytetrafluoroethylene in theform of expanded mesh, fabric of graphite fibers, netting of variouspolymers such as polypropylene or oriented polyethylene, and fiberglass.Such sheet-like member can be large or small in its dimensions, andfollowing preparation of the article having two sheets of a fluorinatedpolymer, said polymer having sulfonyl groups in melt fabricable form,adhered to opposite sides of such reinforcing member, it can be in flatform or cut and/or shaped into other desirable forms such as smallcylinders or saddles, or merely rolled in spiral form for insertion intoa reactor such as a tube or column. Especially when shaping into suchforms is desirable, support members having some degree of stiffness aredesirable. After hydrolysis of the sulfonyl groups to sulfonate groupswith a base and subsequent acidification to form sulfonic acid groups,articles of this kind are useful as a catalyst for catalyzing anychemical reaction catalyzed by hydrogen ion or a strong acid, e.g.,alkylation of aromatic compounds such as the reaction of benzene andethylene at 160° C. to form ethylbenzene. Said hydrolysis with a baseand acidification to form sulfonic acid groups can be carried out eitherbefore or after said cutting, shaping into desirable forms, or rollinginto spiral form. For preparation of such articles with the process andapparatus disclosed herein, the reinforcing member can be of anyconfiguration so long as air can be removed from between the two filmsand from within the reinforcing member through the edge-conveyor belts.Any reinforcing member having raised portions on its surface, on eithera macroscopic of microscopic scale, is suitable. For example, suchmembers can be porous, or have an open-cell structure. Sheets which arenonporous in the thickness direction, but which have surface roughnessin a random or repeating pattern such as projections or furrows, so longas the air can be removed from between the two films, can also be usedas the reinforcement. The fluorinated polymer having sulfonyl groupsused in articles used as a catalyst is preferably a perfluorinatedpolymer. Catalysts made by the process disclosed herein make highlyefficient use of the polymer by virtue of the polymer being largely inthe form of a uniform thin layer.

For use in ion exchange applications and in cells, for example achloralkali cell for electrolysis of brine, the membrane should have allof the functional groups converted to ionizable functional groups.Ordinarily and preferably these will be sulfonic acid and carboxylicacid groups, or alkali metal salts thereof. Such conversion isordinarily and conveniently accomplished by hydrolysis with acid orbase, such that the various functional groups described above inrelation to the melt-fabricable polymers are converted respectively tothe free acids or the alkali metal salts thereof. Such hydrolysis can becarried out with an aqueous solution of a mineral acid or an alkalimetal hydroxide. Base hydrolysis is preferred as it is faster and morecomplete. Use of hot solutions, such as near the boiling point of thesolution, is preferred for rapid hydrolysis. The time required forhydrolysis increases with the thickness of the structure. It is also ofadvantage to include a water-miscible organic compound such asdimethylsulfoxide in the hydrolysis bath.

Before carrying out the hydrolysis described in the previous paragraph,it is also possible to chemically modify one surface of the membrane bymeans other than hydrolysis. This can be especially desirable in thecase of a membrane all of whose component films are prepared frompolymers which contain sulfonyl functional groups and which is intendedto be used in a chloralkali cell. In such a case, it is possible totreat one surface of the membrane whose functional groups are still insulfonyl fluoride or sulfonyl chloride form with a mono- orpolyfunctional amine, such as butylamine or ethylenediamine. Suchtechniques are described in, for example, U.S. Pat. Nos. 4,085,071 and3,969,285. Following amine treatment the membrane is then subjected to ahydrolysis treatment such as that described in the paragraph immediatelyabove.

The various copolymers used in the blends described herein should be ofhigh enough molecular weight to produce tough films in both themelt-fabricable precursor form and in the hydrolyzed ion-exchange form.

To further illustrate the innovative aspects of the present invention,the following examples are provided.

EXAMPLE 1

A membrane of symmetrical cross-section was prepared on apparatus asdescribed hereinabove as follows.

A fabric woven from perfluorocarbon polymer monofilaments wasencapsulated between a pair of similar films of fluorinated polymercontaining sulfonyl groups. The films were each of a copolymer ofperfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) andtetrafluoroethylene having an equivalent weight of 1150 and of an equalthickness of 2 mils (51 micrometers). The fabric was woven fromperfluorocarbon polymer monofilament of 200 denier (0.005 inchesdiameter, or 0.127 mm) having a thread count of 34-38 per inch (13-15per cm) in the warp direction and 17-18 per inch (6-7 per cm) in thefill or weft direction.

During lamination the films and fabric forming the web were passedbetween chevron-shaped heater plates, each of 6 inch (0.15 meter)effective heated length maintained at 460° C. (heater temperaturemeasured by thermocouple) at a rate of 12 inches/minute (5.08×10⁻³meters/sec.). The amount of vacuum applied at the fabric edges throughthe perforated edge-conveyor belt was 26.5 inches of mercury belowatmospheric pressure, or 11.5×10³ Pa absolute pressure.

This laminate was then subjected to a post-treatment with a solution ofethylene diamine on one face only so as to form a catholyte barrierlayer to enhance its efficiency when used as an ion-exchange membrane ina chlor-alkali cell. The treating solution contained 20 ml of ethylenediamine in 80 ml of dimethylsulfoxide which had previously been driedover a molecular sieve of 4 A (Angstrom) size. The membrane was treatedfor 90 minutes at room temperature in this solution. The depth oftreatment penetration was 0.3 mils (7.5 microns) as measured by staininga cross-section of the membrane with Merpacyl orange R dye which showedup the modified surface layer.

After water-washing and hydrolysis, the membrane was tested in achlor-alkali cell for 54 days of continuous operation. The averagevoltage observed was 3.85 volts at an average current efficiency of 90%,equivalent to a power consumption of 2857 KW-hr/metric ton of NaOH whenproducing NaOH of 29% concentration at a current density of 31amps/decimeter² (2 ASI). Other test conditions were as follows: theinlet brine (less than 0.1 ppm calcium) was saturated and the rate wasadjusted to yield an exit brine concentration of 240 gms/liter. Anolytetemperature was 80° C.

EXAMPLE 2

The same apparatus and reinforcement fabric were used as in Example 1.To one side of the fabric was applied a 51-micrometer (2-mil) film of acopolymer of perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) andtetrafluoroethylene having an equivalent weight of 1100. To the otherside of the fabric was applied a pair of pre-blocked films as follows:one film was a 51-micrometer film as just described and the other filmwas a 25-micrometer (1-mil) film of a melt extruded blend of 75% by wt.of a copolymer of methyl perfluoro(4,7-dioxa-5-methyl-8-nonenoate) andtetrafluoroethylene having an equivalent weight of 1050 and 25% by wt.of a copolymer of perfluoro(3,6-dioxa-4-methyl-7-octenesulfonylfluoride) and tetrafluoroethylene having an equivalent weight of 1100;the films were pre-blocked by passing them simultaneously between a pairof nip-rolls to press them together, each film being separately trainedaround the periphery of its respective nip-roll so that the films didnot touch until the nip was reached, so as to achieve a rolling wedgeeffect which precludes entrainment of air between the films, because anyentrained air would lead to formation of bubbled, deformed areas duringformation of the membrane as a result of expansion of the air duringheating; the resulting blocked "bi-film" was used in formation of themembrane with the sulfonyl fluoride film placed against the reinforcingfabric. During lamination, the chevron-shaped heaters at the side havingthe 76-micrometer "bi-film" were maintained at 440° C., and on the otherside at 420° C. (both heater temperatures measured by thermocouple), theline speed was 5.1 mm/sec, and the vacuum at the edge conveyors was 27.5inches of mercury below atmospheric pressure (8.2 KPa absolutepressure). The resultant laminate was hydrolyzed in a caustic solutionand then tested in a chloralkali cell. After 15 days of continuousoperation, the voltage was 3.68 volts, and the current efficiency 98.7%,while producing 30-32% NaOH at 31 amps/decimeter² at 80° C., with anNaCl exit brine concentration of 230-235 gm/liter and an anolyte pH of4.2.

EXAMPLE 3

A membrane was prepared by laminating a 0.051-mm (2-mil) film of acopolymer of methyl perfluoro(4,7-dioxa-5-methyl-8-nonenoate) andtetrafluoroethylene having an equivalent weight of 1050 to each side ofthe same reinforcement fabric as described in Example 1. Lamination wascarried out in the same apparatus at a line speed of 5.1 mm/sec with theheaters in each bank at 400° C., and at a vacuum at the edge conveyorsof 24 inches of mercury below atmospheric pressure (20×10³ Pa absolutepressure). After hydrolysis of the resulting membrane to the carboxylatepotassium salt form, it was tested in a chloralkali cell under thestandard operating conditions as detailed in Example 2, and a voltage of4.06 volts was observed at 98.3% current efficiency after 26 days ofcontinuous operation.

EXAMPLE 4

A membrane was prepared by laminating a 0.025-mm (1-mil) film amelt-extruded blend of 75% by wt. of a copolymer of methylperfluoro(4,7-dioxa-5-methyl-8-nonenoate) and tetrafluoroethylene havingan equivalent weight of 1050 and 25% by wt. of a copolymer ofperfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) andtetrafluoroethylene having an equivalent weight of 1100 to each side ofthe same reinforcement fabric as described in Example 1. Lamination wascarried out in the same apparatus at a line speed of 5.1 mm/sec with theheaters in each bank at 420° C., and at a vacuum at the edge conveyorsof 27 inches of mercury below atmospheric pressure (9.9×10³ Pa absolutepressure). After hydrolysis, this membrane was tested in a chloralkalicell under the standard conditions of Example 2, and a voltage of 3.66volts at 94.1% current efficiency was observed after 14 days ofcontinuous operation.

EXAMPLE 5

In the apparatus of Example 1, two 0.051-mm (2-mil) films of a copolymerof perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) andtetrafluoroethylene having an equivalent weight of 1100 were broughtinto contact with a non-woven uncalendered expanded sheet of titaniummetal (titanium sheet having a thickness of 2.8 mils, or 0.071 mm,expanded to have openings 9.8 mils, or 2.48 mm, by 4.9 mils, or 1.245mm), one film on each side thereof. The chevron heaters were set at 475°C., and a line speed of 24 inches per minute (10.16×10⁻³ m/sec.) andvacuum at the edge conveyors of 24 inches of mercury below atmosphericpressure (20×10³ Pa absolute pressure) were used. The expanded metal webprovides a relatively stiff support member for the films, and followinghydrolysis of the sulfonyl fluoride groups to sulfonate groups with acaustic solution and subsequent acidification to form sulfonic acidgroups, the article so prepared, or after subsequent cutting and/orshaping, is useful as a catalyst for various chemical reactions.

INDUSTRIAL APPLICABILITY

With the process and apparatus described, ion exchange membranes havinggeneral utility for ion exchange purposes are prepared. Such includesselection permeation or absorption of cations, and reverse osmosis.

A specific use for the membranes is in a chloralkali cell such asdisclosed in German patent application No. 2,251,660 published Apr. 26,1973, and Netherlands patent application 72.17598 published June 29,1973. In a manner similar to that disclosed therein, a conventionalchloralkali cell is employed with the distinction of using the novelmembrane described herein to separate the anode and cathode portions ofthe cell.

The web supported membrane of the present invention possesses numeroustechnical advantages over previously known membranes. This membranepossesses exeptional uniformity. The warp and fill strands of thesupport fabric are well covered on both sides of the membrane bypolymer. Prior art membranes made with thin layers of fluorinatedpolymer invariably had either bare junctions or junctions covered byonly a thin polymer layer, and were thus vulnerable to abrasion andexposure of the junctions of the strands; this invariably led to ruptureof the membrane and a short useful life. In the present membrane, thestrands and the junctions are well covered by polymer on both sides andthus have greater resistance to abrasion and a longer useful life.Furthermore, the greater uniformity of polymer filling the window areasof the fabric, and the consequent substantial elimination of windowareas having only a thin layer of polymer therein, results in theelimination of nonuniform current gradients which lead to more rapiddeposition of calcium impurities in the thin areas, which in turn leadsto rupture of the thin areas and short membrane life. Additionally, thegreater uniformity of the surface contours of the membrane of thisinvention and the elimination of deep recesses in the window areassubstantially reduces the tendency for gas blinding of membrane duringuse in a chloralkali cell. Accordingly, the total thickness of thepolymeric material can be substantially reduced over prior artmembranes, with consequent improvement in efficiency and reduction ofvoltage when employed in electrolytic processes such as the chloralkaliprocess. Additionally, the membranes of the invention perform wellduring chloralkali electrolysis when used under conditions of highdepletion of the feed brine. Furthermore, the surface hydrolysis step ofthe prior art process for making membranes is eliminated. Also, theprocess eliminates the step of stripping a membrane from a laminatingdrum or other support surface which in many cases led to a membranehaving one support textured or embossed by the pattern of the supportsurface.

I claim:
 1. Apparatus for making a reinforced membrane, comprising,(1) aframe, and mounted on said frame, (2) means for guiding at least twocontinuous webs of film and a continuous web of reinforcing materialinto face-to-face contact such that proximate surfaces of two of saidwebs of film contact opposite sides of said web of reinforcing material,(3) two sets of flexible endless belts which cooperate to engageopposite sides of the resulting assembly of said webs at the edgeportions thereof and to transport said assembly, each set consisting oftwo belts, one belt of each set having a series of perforations alongits entire length, each set of belts extending beyond an edge of saidassembly, and guide means for said belts, (4) vacuum means including twovacuum manifolds, one manifold adjacent each said perforated belt, forremoving air from between said films of said assembly at the edgeportions thereof through said perforations, (5) two banks of heaters,one bank adjacent each exposed film surface, each bank consisting of aplurality of heaters disposed for heating first the center portion ofsaid assembly and then progressively toward and including the edgesthereof as said assembly is transported therebetween, to fuse saidassembly into a reinforced membrane, (6) means for guiding said assemblybetween said banks of heaters, (7) a wind-up for collecting saidreinforced membrane, and (8) means for driving said wind-up.
 2. Theapparatus of claim 1 wherein said heaters are radiant heaters.
 3. Theapparatus of claim 2 wherein said heaters are arranged in achevron-shaped array, disposed such that said assembly of webs is firstheated in the center portion thereof.
 4. The apparatus of claim 3wherein said guide means for said belts includes for each said belt oneguide which is moveable toward and away from said belt for increasing ordecreasing the tension on said belt.